1. Field of the Invention
This invention relates to a method for forming an energy subtraction image wherein a parameter is optimized which is used when subtraction processing, in particular, digital subtraction processing, is carried out on radiation images, and an apparatus for carrying out the method.
2. Description of the Prior Art
Techniques for carrying out digital subtraction processing on radiation images have heretofore been known. When digital subtraction processing is to be carried out, two radiation images recorded under different conditions are photoelectrically read out, and digital image signals which represent the radiation images are obtained. The image signal components of the digital image signals which represent corresponding picture elements in the radiation images are then subtracted from each other with a predetermined parameter, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, two digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure can be reproduced from the difference signal.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order to extract the image of a specific structure of an object from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, such characteristics are utilized that a specific structure of an object exhibits specific radiation energy absorbing characteristics. Specifically, an object is exposed to several kinds of radiation with different energy distributions. In this manner, two radiation images, in which different images of a specific structure are embedded, are obtained. Thereafter, the image signals representing the two radiation images are weighted appropriately, i.e. an appropriate value of the parameter for a subtraction process is determined, and the weighted image signals are subjected to a subtraction process. The image of the specific structure is thereby extracted.
Subtraction processing is extremely effective, particularly for medical diagnosis utilizing image processing of X-ray images, and electronics research has continued to develop improved subtraction processing methods.
By carrying out energy subtraction processing on radiation images of an object constituted of bones and soft tissues, a subtraction image can be obtained in which the patterns of the bones have been erased and only the patterns of the soft tissues have been extracted. (Such a subtraction image will hereinbelow be referred to as the "soft tissue image".) Alternatively, a subtraction image can be obtained in which the patterns of the soft tissues have been erased and only the patterns of the bones have been extracted. (Such a subtraction image will hereinbelow be referred to as the "bone image".) However, in such cases, the value of the parameter is not necessarily suitable for every subtraction process or for the whole area of the image. This is presumably because the level of radiation energy fluctuates due to, for example, fluctuations in the X-ray tube voltage for different exposures to the radiation, and the effective radiation absorption coefficient varies for parts of the object having different thicknesses due to non-monochromaticity of the X-rays.